It is common practice to represent a color by three signals, either analog or digital, which correspond to three primaries. These primaries may be any one of a number of suitable triplets of colors such as red-green-blue or cyan-magenta-yellow. The red-green-blue triplet is in common use for color video signals. Different colors are represented by different combinations of intensities of these three primaries. It is often necessary to adjust the colors corresponding to triplets of intensities in order, for example, to correct for a color cast or to adjust the overall brightness.
The common method for performing such an adjustment is to modify each of the three signals comprising the color signal independently. In an analog system this is accomplished by passing each of the three signals through its own non-linear transformation circuit. This prior art method is illustrated in FIG. 1, wherein the triplet colors Rin, Gin, and Bin are transformed in the R, G, and B transforms, respectively, to form the output signals Rout, Gout, and Bout, respectively. In a digital system each signal is modified by its own, independent look-up table. The look-up table contains the new value for each possible input value and replaces the transform blocks of FIG. 1.
In U.S. Pat. No. 4,712,132, entitled "Device and Method for Reducing the Amplitude Range of Signals Representing an Image", by S. Soca there is disclosed, for example, in FIG. 2, a device that operates upon three input signals to obtain three output signals the values of which retain the same ratios as the input signals.
It is the purpose of this invention to propose a new method which adjusts only the brightness of the input color while not affecting its color. This new method can be embodied in an analog circuit, digital hardware, or be computed explicitly by a general-purpose computing device mimicking the digital hardware.